In recent years, radio frequency IC tags have become widely used for information management and distribution management of commodities, articles, and the like. Moreover, these radio frequency IC tags are also coming into common use for the identification and management of animals. Such radio frequency IC tags usually include a small IC chip on which information is stored, and a small antenna for transmitting information stored in the IC chip by, such as by radio frequency transmission, other electromagnetic energy transmission, or the like, as is known in the art. (See, for example, U.S. Pat. No. 6,617,172 to Usami, and U.S. patent application Ser. No. 10/513,995, to Oozeki, filed Sep. 3, 2003, both of which are incorporated by reference herein in their entireties.) To be more specific, such a radio frequency IC tag may be equipped with a small IC chip having a width of about 0.4 mm, a length of about 0.4 mm, and the height of about 0.1 mm, for example, in proximity to the central portion of a rectangular antenna.
The radio frequency IC tag is attached to a commodity, animal, or the like, in actual use. Therefore, if a reader is put over the radio frequency IC tag, it is possible to read out, in a non-contact manner, information that is to the IC chip (that is, information about, for example, attributes of an individual commodity or those of an individual animal) To attach such a radio frequency IC tag to a commodity or an animal, it is desirable that the radio frequency IC tag be configured as small as possible. For this purpose, it is necessary to reduce the size of an antenna of the radio frequency IC tag.
FIGS. 12A, 12B are diagrams each illustrating a configuration of a dipole antenna in the conventional radio frequency IC tag. As shown in FIG. 12A, an IC chip 17 is mounted on the central part of a dipole antenna 21. The length L′ of the dipole antenna 21 is λ/2 so that the dipole antenna 21 can achieve the maximum antenna efficiency. Here, λ denotes the wavelength in the frequency to be used. Therefore, under the prior art, in order to reduce the size of the radio frequency IC tag, for example, part of the dipole antenna 21 might be cut off for use, as shown in FIG. 12B.
However, because the maximum antenna efficiency is obtained when the antenna length L′ of the dipole antenna is λ/2. Therefore, if as shown in FIG. 12B the antenna is cut off in the longitudinal direction to miniaturize the radio frequency IC tag, the antenna efficiency decreases, which causes the communication distance to be shortened, resulting in difficult communications. Consequently, it may become impossible to read out information stored in the IC chip. FIG. 13 is a diagram representing characteristics of a dipole antenna having a shape as shown in FIGS. 12A-12B, and illustrates the relationship between the antenna length L′ (in mm) and the communication distance S (in mm). As shown in FIG. 13, if the antenna length L′ is shortened, the communication distance S (mm) rapidly decreases. For example, if the antenna width D′ is fixed at a value of 1.5 mm, the graph of FIG. 13 illustrates that it becomes impossible to carry out communications when the antenna length L′ becomes 25 mm or less.
In addition, a technique used in microstrip antennas is also known. In this technique, the central part of an antenna is narrowed, and both end parts extending along the direction in which an electric current flows are widened. In other words, the antenna is configured as an H-shaped antenna to reduce the size of the antenna. This technique is intended for reducing the size of antennas by concentrating the magnetic field in a thin constriction of the H type antenna to increase the inductance so that a resonance frequency is decreased. (See, for example, Japanese Patent Laid-Open No. 2001-53535, paragraph numbers 0015 through 0023, and FIGS. 1 and 2, which is hereinafter referred to as “Patent document 1”.) Further, a technique used in non-contact IC tags using an H-shaped antenna is also known. This technique achieves a reduction in the size of non-contact IC tags by applying ohmic contact to a connected part of an H type antenna. (See, for example, Japanese Patent Laid-Open No. 2003-243918, paragraph numbers 0012 through 0027, and FIGS. 1 through 4, which is hereinafter referred to as “Patent document 2”.)
However, the H type antenna disclosed in the above-mentioned patent document 1 is a so-called microstrip antenna comprising an H-shaped patch electrode formed on a surface of a dielectric and a ground electrode formed on the back surface thereof. The microstrip antenna is utilized as a small-size antenna for cellular phones. The microstrip antenna cannot be applied to a strip antenna having no ground electrode as it is. Further, what is realized in the antenna disclosed in the above-mentioned patent document 2 is a non-contact IC tag that is intended for the miniaturization by use of an H-shaped antenna. However, when the H-shaped antenna is configured by use of two wide antenna patterns that are symmetrically formed on a substrate, and an IC tag label that bridges these antenna patterns, the antenna patterns and the IC tag label are connected in ohmic contact.
To be more specific, in a manufacturing process of the non-contact IC tag, the IC tag label is directly mounted on the antenna patterns to make an ohmic contact in a state in which film coating is not performed on the substrate on which the antenna patterns are formed. The technique in the patent document 2, therefore, cannot coat with a film between the two wide antenna patterns and the antenna patterns equipped with the IC chip to bridge in the manufacturing process of the H type antenna. In other words, a non-contact IC tag that configures an H type antenna by means of electrostatic capacitive coupling cannot be manufactured by utilizing the known conventional manufacturing processes.